when I study microfluidic paper-based to detemination of nitrite, coffer-ring effect is my problem but I don't know how to do that to stop. I have attached the image below
1. Use a surfactant: There are quite a few options but interaction has to to be evaluated. Since it looks like a Griess Assay, one wouldn't want to add any other chemicals to avoid possible interaction with the assay.
2. Aid drying: Improving the drying rate (probably using a hot air oven at slightly elevated i.e. more than room temperatures) could significantly decrease the coffee ring effect.
Thank Sohan Dudala about your answers, I will try use a surfactant. I try aid drying but it can't decrease coffering effect in my test. I try prepare Griess Assay in ethanol to improving the drying rate, it can't too.